Hydrometallurgical process for nickel oxide ore

ABSTRACT

Provided is a hydrometallurgical process for nickel oxide ore using high pressure acid leaching to be capable of achieving improvement of durability of production facilities, simplification of the production facilities, suppression of the cost and environmental risks caused by compression of the volume of a tailings dam that stores wastes, and separation and recovery of impurity components which can be utilized as a resource. The hydrometallurgical process for nickel oxide ore of recovering nickel and cobalt using the high pressure acid leaching is characterized by including at least one step selected from step (A) (a step of separating and recovering chromite particles), step (B-1) (a step of neutralizing a leachate with a magnesium-based neutralizing agent), and step (B-2) (a step of neutralizing a leach residue slurry with the magnesium-based neutralizing agent).

BACKGROUND

1. Field of the Invention

The present invention relates to a hydrometallurgical process for nickeloxide ore, and relates to a hydrometallurgical process for nickel oxideore of recovering nickel and cobalt from nickel oxide ore by a highpressure acid leaching that includes an ore processing step, a leachingstep, a solid-liquid separation step, a neutralization step, a zincremoval step, a sulfurization step and a final neutralization step. Thehydrometallurgical process can achieve the tasks of suppressing theabrasion of facilities such as piping and pumps caused by the ore slurryproduced from the ore processing step, increasing durability, reducingthe amount of a final neutralized residue produced from the finalneutralization step, and suppressing the cost and environmental risks bycompressing the volume of the tailings dam that stores the leachingresidue that will be disposed, neutralized precipitate, and the like,and also enables separation and recovery of impurity components that canbe effectively utilized by recycling.

2. Description of the Related Art

In recent years, as a result of further progress in oligopolization ofthe rights to mining for mineral resources such as coal, iron, copper,nickel, cobalt, chromium, and manganese, the raw material cost for metalsmelting is increasing to a large extent. Therefore, even for metalsmelting, since it is disadvantageous in terms of cost, development oftechnologies for using low grade raw materials that have not beenhitherto taken into consideration is underway as a measure for costreduction.

For example, in regard to nickel smelting, materials having excellentcorrosion resistance at a high temperature and a high pressure have beendeveloped. Thus, attention has been paid to a hydrometallurgical processbased on a high pressure acid leaching of subjecting nickel oxide ore toacid leaching with sulfuric acid under pressure.

The high pressure acid leaching does not include dry processes such as areduction process and a drying process, unlike a pyrometallurgicalmethod which is a conventional common smelting method for nickel oxideore, and is advantageous in terms of energy and cost. Therefore, thehigh pressure acid leaching will be continuously considered as apromising technology as a smelting method for low grade nickel oxideore.

Accordingly, in order to increase the level of performance as a smeltingprocess, various suggestions have been made mainly on the leachingprocess at a high temperature under pressure, in connection with anincrease in the leaching ratios of nickel and cobalt, solutionpurification of the leachate, a decrease in the amount of use of theoperation materials, and the like.

Meanwhile, regarding a process of utilizing leaching at a hightemperature under pressure, for example, there has been suggested amethod of recovering valuable metals, such as nickel, cobalt andmanganese, from oxide ores containing these metals, the method includingthe following steps (a) to (c) (see, for example, JP 6-116660 A).

Step (a): Oxide ore that has been slurrified in advance is subjected toleaching at normal pressure under sulfuric acid condition, using apressurized acid leachate obtained in a step (b) as illustrated below,and a normal-pressure leachate and a normal-pressure leach residue areobtained.

Step (b): The normal-pressure leach residue obtained in the step (a) isallowed to react with sulfuric acid in an oxidizing atmosphere at a hightemperature and a high pressure, and thus a pressurized acid leachate isobtained.

Step (c): A neutralizing agent is added to the normal-pressure leachateobtained in the step (a) to neutralize the leachate, subsequently asulfurized alkali compound is added thereto, and nickel and cobalt inthe leachate are recovered as sulfides.

In this method, the leach rate of nickel from ore is increased byperforming two-stage leaching of subjecting an ore slurry tonormal-pressure leaching (step (a)), and then subjecting thenormal-pressure leach residue to acid leaching under pressure (step(b)). At the same time, the excess acid contained in the leachate ofpressurized acid leaching is neutralized by the alkali componentcontained in the normal-pressure leach residue, and the burden of theneutralization step (step (c)) is reduced.

However, due to the two-stage leaching, there is a problem that thenumber of facility items increases so that more cost and efforts areneeded, and it requires expenses to treat a large amount of a thinsolution generated at the time of washing the leach residue.

Thus, in order to solve these problems, there has been suggested amethod including the steps (1) to (4) as illustrated below, as anotherprocess of utilizing leaching at a high temperature under pressure (see,for example, JP 2005-350766 A).

(1) Leaching step: Nickel oxide ore is prepared into a slurry, sulfuricacid is added thereto, the mixture is stirred at a temperature of 220°C. to 280° C., and thus a leached slurry is formed.

(2) Solid-liquid separation step: The leached slurry obtained in theprevious leaching step is washed using multi-stage thickeners, and theleached slurry is separated into a leachate containing nickel andcobalt, and a leach residue.

(3) Neutralization step: The pH of the leachate obtained in thesolid-liquid separation step is adjusted to 4 or less using calciumcarbonate while suppressing oxidation of the leachate, a neutralizedprecipitate containing trivalent iron is produced, and the neutralizedprecipitate is separated into a neutralized precipitate slurry and amother liquor for nickel recovery.

(4) Sulfurization step: Hydrogen sulfide gas is blown into the motherliquor for nickel recovery obtained in the neutralization step, sulfidescontaining nickel and cobalt are produced, and the sulfides areseparated from a barren liquor.

Here, an outline of a practical plant based on the technology disclosedin JP 2005-350766 A will be described with reference to the drawings.

FIG. 2 is a smelting process diagram illustrating an exemplary practicalplant based on the hydrometallurgical process for nickel oxide ore (JP2005-350766 A).

In FIG. 2, nickel oxide ore 8 is first mixed with water to form a mixedliquid, subsequently the removal of foreign material from the mixedliquid and the adjustment of ore particle size are carried out, and anore slurry 9 is formed, in ore processing step (1).

Next, the obtained ore slurry 9 is subjected to high-temperaturepressure leaching using sulfuric acid, thereby a leached slurry 10 isformed, in leaching step (2).

The leached slurry 10 obtained is subjected to solid-liquid separationstep (3) to be washed in multiple stages, and then the leached slurry isseparated into a leachate 11 containing nickel and cobalt, and a leachresidue slurry 12.

The separated leachate 11 is subjected to neutralization step (4), andis separated into a neutralized precipitate slurry 13 containingtrivalent iron hydroxide and a mother liquor (1) 14 for nickel recovery.

The mother liquor (1) 14 is subjected to zinc removal step (5) of addinga sulfurizing agent, and the mother liquor is separated into a zincsulfide precipitate 15 containing zinc sulfide, and a mother liquor (2)16 for nickel recovery.

Meanwhile, the mother liquor (2) 16 is subjected to sulfurization step(6), and is separated into a mixed sulfide containing nickel and cobalt,and a barren liquor 18 having nickel and the like removed therefrom. Thebarren liquor 18 is used as washing water for the leach residue insolid-liquid separation step (3).

Finally, the leach residue slurry 12 is subjected to finalneutralization step (7) together with an excess amount of the barrenliquor 18, and the leach residue slurry is neutralized. A finalneutralized residue 19 is stored in a tailings dam 20.

A feature of this method lies in that by washing the leached slurry inmultiple stages in the solid-liquid separation step, the amount ofneutralizing agent consumption and the amount of precipitate in theneutralization step can be reduced; since the true density of the leachresidue can be increased, the solid-liquid separation characteristicscan be improved; and the process is simplified by performing theleaching step simply by high-temperature pressure leaching. Thus, thismethod is considered to be advantageous against the method suggested inJP 6-116660 A.

Furthermore, it is believed that when such a barren liquor is used asthe washing liquid used in the solid-liquid separation step, nickeladhering to the leach residue can be leached and recovered usingresidual sulfuric acid, and repeated use of water can be carried outeffectively and efficiently.

Furthermore, when the neutralized precipitate slurry is sent to thesolid-liquid separation step, the loss of nickel can be reduced, andtherefore, it is believed to be more advantageous.

However, a practical plant adopting this method has the followingproblems.

First, suppression of abrasion of facilities is exemplified.

Although nickel oxide ore is conveyed in the form of slurry betweenvarious processes, abrasion of the facility materials is markedlyaccelerated by the conveyed slurry so that there occurs a high frequencyof maintenance particularly in facilities such as piping and pumps inthe leaching step, and this high frequency serves as a major factor foran increase in the maintenance cost and a decrease in the rate of plantoperation.

Secondly, reduction of amount of final neutralized residue isexemplified.

The leach residue obtained in the solid-liquid separation step iscombined with excess barren liquor produced from the sulfurization step,and the mixture is made harmless by a neutralization treatment of addinglimestone slurry or a slaked lime slurry thereto.

The final neutralized residue produced from this final neutralizationtreatment step (hereinafter, may be referred to as final neutralizationstep) is stored in the tailings dam. However, the final neutralizedresidue contains, in addition to impurity components such as hematiteand chromite in the leach residue, gypsum that is formed by theneutralization treatment so that the final neutralized residue cannot berecycled, and there is a heavy burden of expenses for the constructionand maintenance management of the tailings dam.

Therefore, there has been a demand for a solution for the problemsdescribed above with regard to the practical plant using ahydrometallurgical process based on the conventional high pressure acidleaching.

Furthermore, in order to solve the problems in an effective andeconomically efficient manner, it is an effective measure to efficientlyseparate and recover impurity components that are contained in the oreor leach residue, and it is also demanded to recycle and effectivelyutilize these impurity components.

Thus, the applicant of the present application has suggested in JP2005-350766 A a hydrometallurgical process for nickel oxide ore, whichincludes a step of physically separating and recovering particlescontaining at least one selected from silica mineral, chromite orsilica-magnesia mineral from ore slurry, and a step of physicallyseparating and recovering hematite particles in the leach residueslurry, in a hydrometallurgical process based on a high pressure acidleaching. However, improvements have been further needed for efficientseparation and recovery of impurity components contained in ore or leachresidue.

Under such circumstances, the invention was achieved in view of theproblems of the conventional technologies, and it is an object of theinvention to provide a hydrometallurgical process for nickel oxide oreof recovering nickel and cobalt using a high-pressure acid leaching thatincludes an ore processing step, a leaching step, a solid-liquidseparation step, a neutralization step, a zinc removal step, asulfurization step, and a final neutralization step. Thehydrometallurgical process can achieve the tasks of suppressing theabrasion of facilities such as piping and pumps caused by the ore slurryproduced from the ore processing step, increasing durability, increasingthe solid content ratio of the ore slurry, simplifying the facilities ofthe ore processing step, reducing the amount of a final neutralizedresidue produced from the final neutralization step, and suppressing thecost and environmental risks by compressing the volume of the tailingsdam that stores the leaching residue that will be disposed, neutralizedprecipitate, and the like, and also enables separation and recovery ofimpurity components, such as chromite and hematite, that can beeffectively utilized by recycling.

SUMMARY

In order to achieve the object described above, the inventors of theinvention conducted extensive investigations on the solution for theproblems described above, in connection with a hydrometallurgicalprocess for recovering nickel and cobalt from nickel oxide ore by a highpressure acid leaching that includes an ore processing step, a leachingstep, a solid-liquid separation step, a neutralization step, a zincremoval step, a sulfurization step, and a final neutralization step. Asa result, the inventors carried out at least one step selected from step(A) of separating and recovering, by a particular method, particlescontaining chromite in an ore slurry produced from an ore processingstep; and step (B) of performing, after the step (A), and after aleaching step and a solid-liquid separation step, neutralization by aparticular method that does not produce gypsum, and recovering themetals, and the inventors found that this method is effective as asolution for the problems described above, thus completing theinvention.

That is, according to a first aspect of the invention, there is provideda hydrometallurgical process for nickel oxide ore of recovering nickeland cobalt using a high pressure acid leaching that includes an oreprocessing step, a leaching step, a solid-liquid separation step, aneutralization step, a zinc removal step, a sulfurization step, and afinal neutralization step, the hydrometallurgical process including atleast one step selected from the following step (A), step (B-1), andstep (B-2):

Step (A): a step of separating and recovering chromite particles in anore slurry produced in the ore processing step, by a recovery processincluding a specific gravity separation;

Step (B-1): a step of neutralizing a leachate with a magnesium-basedneutralizing agent, the leachate being produced by subjecting the oreslurry that has a chromium grade lowered after the step (A), to theleaching step and the solid-liquid separation step; and

Step (B-2): a step of neutralizing a leach residue slurry with amagnesium-based neutralizing agent to recover hematite particles, theleach residue slurry being produced by subjecting the ore slurry thathas a chromium grade lowered after the step (A), to the leaching stepand the solid-liquid separation step.

A second aspect of the invention is a hydrometallurgical process fornickel oxide ore of recovering nickel and cobalt from nickel oxide oreusing a high pressure acid leaching that includes an ore processingstep, a leaching step, a solid-liquid separation step, a neutralizationstep, a zinc removal step, a sulfurization step, and a finalneutralization step, the method including the following step (A), step(B-1), and step (B-2).

Step (A): a step of separating and recovering chromite particles in anore slurry produced in the ore processing step, by a recovery processincluding a specific gravity separation;

Step (B-1): a step of neutralizing a leachate with a magnesium-basedneutralizing agent, the leachate being produced by subjecting the oreslurry that has a chromium grade lowered after the step (A), to theleaching step and the solid-liquid separation step; and

Step (B-2): a step of neutralizing a leach residue slurry with amagnesium-based neutralizing agent to recover hematite particles, theleach residue slurry being produced by subjecting the ore slurry thathas a chromium grade lowered after the step (A), to the leaching stepand the solid-liquid separation step.

A third aspect of the invention is the hydrometallurgical process fornickel oxide ore according to the first and second aspects, wherein therecovery process of the step (A) includes subjecting the ore slurry tocyclone classification, reducing fine iron hydroxide particles, and thenrecovering chromite particles in the ore slurry from the ore slurry as aconcentrate of chromite using the specific gravity separation.

A fourth aspect of the invention is the hydrometallurgical process fornickel oxide ore according to the third aspect, wherein the recoveryprocess of the step (A) includes performing cyclone classificationwithout diluting the slurry concentration of the ore slurry.

A fifth aspect of the invention is the hydrometallurgical process fornickel oxide ore according to the first to fourth aspects, wherein therecovery process of the step (A) includes collecting chromite into anunderflow in cyclone classification in the entire amount except forunavoidable losses.

A sixth aspect of the invention is the hydrometallurgical process fornickel oxide ore according to the first to fifth aspects, wherein thespecific gravity separation includes at least one step of selected froma step of using a density separator and a step of using a spiralconcentrator.

A seventh aspect of the invention is the hydrometallurgical process fornickel oxide ore according to the sixth aspect, wherein the step ofusing the density separator is performed two times or more onconcentrated slurry using the density separator.

An eighth aspect of the invention is the hydrometallurgical process fornickel oxide ore according to the sixth aspect, wherein the step ofusing the spiral concentrator is performed two times or more onconcentrated slurry using the spiral concentrator.

A ninth aspect of the invention is the hydrometallurgical process fornickel oxide ore according to the sixth aspect, wherein a pulp contentof a slurry that is supplied to the spiral concentrator is 15 wt %solids to 35 wt % solids, preferably 20 wt % solids to 30 wt % solids.

A tenth aspect of the invention is the hydrometallurgical process fornickel oxide ore according to the sixth aspect, wherein an amount ofteeter water supplied to the density separator is 0.5 to 7.0[m3·h−1/m2].

An eleventh aspect of the invention is the hydrometallurgical processfor nickel oxide ore according to the first to tenth aspects, whereinafter the specific gravity separation, the slurry is subjected to amagnetic separation, which is a physical separation, to remove hematite,and non-magnetized material is then recovered as a chromite concentrate.

A twelfth aspect of the invention is the hydrometallurgical process fornickel oxide ore according to the first and second aspects, wherein inthe step (B-2), a pH of the leach residue slurry neutralized is adjustedto be in a range of 4 to 7, and thereafter, final neutralization iscarried out using an alkali other than a magnesium-based neutralizingagent.

A thirteenth aspect of the invention is the hydrometallurgical processfor nickel oxide ore according to the first to third aspects, wherein inthe step (B-2), the leach residue slurry or a neutralized residue slurryincluding the leach residue slurry is subjected to cycloneclassification, and a fine particle portion thus classified is recoveredas a concentrate of hematite.

A fourteenth aspect of the invention is the hydrometallurgical processfor nickel oxide ore according to the first to thirteenth aspects,wherein the ore processing step is a step of performing removal offoreign material in mined raw material ore and adjustment of the oreparticle size to form an ore slurry; the leaching step is a step ofadding sulfuric acid to the ore slurry and stirring the mixture at ahigh temperature and a high pressure to form a leached slurry that iscomposed of a leach residue and the leachate; the solid-liquidseparation step is a step of washing the leached slurry in multiplestages to obtain the leachate containing nickel and cobalt, and theleach residue slurry; the neutralization step is a step of adding analkali to the leachate, to form a neutralized precipitate slurrycontaining trivalent iron, and a mother liquor for nickel recovery; thezinc removal step is a step of blowing in hydrogen sulfide gas into themother liquor for nickel recovery to form a zinc sulfide precipitateslurry and a mother liquor for nickel and cobalt recovery; thesulfurization step is a step of blowing in hydrogen sulfide into themother liquor for nickel and cobalt recovery, and producing a mixedsulfide containing nickel and cobalt, and a barren liquor; and the finalneutralization step is a step of adding an excess of the barren liquorto the leach residue slurry and adjusting the pH of the mixture to beabout 8 to 9, to obtain a final neutralized residue.

A fifteenth aspect of the invention is the hydrometallurgical processfor nickel oxide ore according to the first to fourteenth aspects,wherein the adjustment of a particle size of the ore in the oreprocessing step is carried out by screening to a particle size of 2 mmor less.

A sixteenth aspect of the invention is the hydrometallurgical processfor nickel oxide ore according to the first to fifteenth aspects,wherein a grade of chromium(III) oxide in the concentrated chromite is41 wt % or more.

According to the hydrometallurgical process for nickel oxide ore of theinvention, when step (A) and step (B) are adopted in ahydrometallurgical process of recovering nickel and cobalt from nickeloxide ore by a high pressure acid leaching that includes an oreprocessing step, a leaching step, a solid-liquid separation step, aneutralization step, a zinc removal step, a sulfurization step, and afinal neutralization step, the conventional problems described above canbe solved as below. Therefore, the industrial value of thehydrometallurgical process is enormous.

When the step (A) of the invention is adopted, particles containingchromite in the ore slurry that is produced in the ore processing stepare separated and recovered, so that abrasion of facilities such aspiping and pumps at the time of transportation of the ore slurry can besuppressed.

Further, since chromite is separated before wet smelting, reduction ofthe amount of leach residue can be expected, and the amount of a finalneutralized residue can be reduced. Furthermore, when the chromite thusseparated can be concentrated, the concentrated chromite can also beeffectively utilized as a resource.

When the step (B) of the invention is adopted, since hematite in theleach residue that is produced in the solid-liquid separation step isseparated and recovered, reduction of the amount of a final neutralizedresidue that is produced from the final neutralization step can bepromoted, the cost and environmental risks caused by compression of thevolume of the tailings dam that stores the leach residue that will bedisposed, a neutralized precipitate, and the like, can be suppressed,and also, the hematite that has been separated and recovered can beeffectively utilized as a resource for iron.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a flowchart of an embodiment of a hydrometallurgical processfor nickel oxide ore according to the invention.

FIG. 2 is a flowchart of a practical plant based on a conventionalhydrometallurgical process for nickel oxide ore (JP 2005-350766 A).

FIG. 3 is an exemplary flow diagram of Example 1 of the invention.

FIG. 4 is an exemplary flow diagram of Example 2 of the invention.

FIG. 5 is an exemplary flow diagram of Comparative Example 1 of theinvention.

FIG. 6 is an exemplary flow diagram of Comparative Example 4 of theinvention.

DETAILED DESCRIPTION

The hydrometallurgical process for nickel oxide ore of the invention isa hydrometallurgical process of recovering nickel and cobalt from nickeloxide ore by a high pressure acid leaching that includes an oreprocessing step, a leaching step, a solid-liquid separation step, aneutralization step, a zinc removal step, a sulfurization step, and afinal neutralization step, the hydrometallurgical process including atleast one step selected from the following step (A), step (B-1), andstep (B-2).

[Steps]

Step (A)

This is a step of separating and recovering chromite particles in theore slurry produced from the ore processing step, by a recovery processincluding a specific gravity separation.

Step (B-1)

This is a step of performing neutralization of a leachate that isobtained by subjecting the ore slurry having a lowered Cr grade throughthe step (A) to a leaching step and a solid-liquid separation step,using an Mg-based neutralizing agent such as Mg(OH)₂ or MgO.

Step (B-2)

This is a step of performing neutralization of a leach residue slurrythat is obtained by subjecting the ore slurry having a lowered Cr gradethrough the step (A) to a leaching step and a solid-liquid separationstep, using an Mg-based neutralizing agent such as Mg(OH)₂ or MgO,thereby recovering hematite particles.

It is important for the process of the invention to include at least onestep selected from the step (A), step (B-1), and step (B-2), in view ofsolving the problems.

Adoption of the step (A) is intended to suppress the abrasion offacilities, such as piping and pumps, at the time of transportation ofthe ore slurry, by separating and recovering particles containingchromite in the ore slurry produced from the previous ore processingstep.

That is, abrasion is suppressed by separating chromite that is generallycontained in nickel oxide ore and has a very high hardness value.Furthermore, by eliminating in advance chromite from the ore slurrybefore wet smelting, reduction of the amount of leach residue isexpected, and the amount of a final neutralized residue may be reduced.

Furthermore, when the separated and recovered chromite can besufficiently concentrated, the chromite can also be effectively utilizedas a resource.

On the other hand, since adoption of step (B) enables separation andrecovery of hematite in the leach residue produced from the solid-liquidseparation step, the amount of the final neutralized residue producedfrom the final neutralization step is reduced, and the expenses andenvironmental risks caused by compression of the volume of the tailingsdam for storing the leach residue to be disposed, a neutralizedprecipitate and the like, can be suppressed. At the same time, thehematite thus separated and recovered can also be effectively utilizedas a resource for iron.

Namely, iron in the nickel oxide ore is hydrolyzed at a high temperaturein the leaching step, and therefore, iron is contained in the form ofhematite in the final neutralized residue. However, since the finalneutralized residue contains gypsum that is formed by a neutralizationtreatment using a neutralizing agent containing Ca, in addition tochromite in the leach residue, the iron grade is as low as 30% to 40% byweight, and it is not feasible to effectively utilize the finalneutralized residue directly as a raw material for iron making or thelike.

It is because sulfur (gypsum; calcium sulfate), chromium (chromite) andthe like that are contained in the final neutralized residue arecomponents that affect the distribution of trace components in pig iron,the quality of steel products, and the like, and it is desirable tosuppress any inclusion of these impurity elements.

On the contrary, in the invention, since neutralization is achievedusing an Mg-based neutralizing agent, MgSO4 having high solubility isproduced, and since sulfur is fixed to solids, a hematite having a lowsulfur-level can be separated and recovered.

Next, an outline of the hydrometallurgical process for nickel oxide oreof the invention will be described with reference to FIG. 1.

FIG. 1 is a smelting flowchart of an exemplary embodiment according tothe hydrometallurgical process for nickel oxide ore related to theinvention.

As illustrated in FIG. 1, first, nickel oxide ore 8 is mixed with waterto form a mixed liquid in ore processing step [1], and then the removalof foreign material from the mixed liquid and the adjustment of the oreparticle size are carried out to form an ore slurry 9.

Thereafter, the ore slurry 9 is subjected to step (A), which is newlyprovided, to separate and recover chromite 23. An autoclave-suppliedslurry 22 on one side is supplied to leaching step [2].

Here, the autoclave-supplied slurry 22 is converted to a leached slurry10 by leaching valuable components such as nickel and cobalt withsulfuric acid using an autoclave or the like.

The leached slurry 10 thus formed is supplied to solid-liquid separationstep [3] that uses multi-stage thickeners or the like, and is separatedinto a leachate 11 containing nickel and cobalt, and a leach residueslurry 12.

The separated leachate 11 is supplied to the step (B-1), and isseparated into a residue 26 after the step (B-1) containing trivalentiron hydroxide as a main component, and a mother liquor (1) 14containing nickel.

The mother liquor (1) 14 is subjected to zinc removal step [5] in whicha sulfurizing agent is added, and is then separated into a zinc sulfideprecipitate 15 containing zinc sulfide, and a mother liquor (2) 16 fornickel recovery.

Subsequently, the mother liquor (2) 16 is subjected to sulfurizationstep [6] in which a sulfurizing agent is added, and is separated into amixed sulfide 17 containing nickel and cobalt, and a barren liquor 18.

Meanwhile, the barren liquor 18 may be used as washing water for theleach residue in the solid-liquid separation step [3], and the barrenliquor 18 may also be supplied to the final neutralization step.

Finally, some of the leach residue slurry 12 is supplied to the step(B-2) together with an excess amount of the barren liquor 18, and issubjected to a neutralization treatment. Thus, hematite 28 is separatedand recovered.

At that time, a treatment solution 27 after the step (B-2), and theother portion of the leach residue slurry 12 that was not supplied tothe step (B-2) are supplied to [7] final neutralization step, and theleach residue slurry is neutralized to about pH 8 to 9.

The final neutralized residue 19 thus obtained is stored in the tailingsdam 20.

Hereinafter, the various steps will be described in more detail.

[1] Ore Processing Step and Step (A)

The ore processing step is a step of forming ore slurry by performingremoval of foreign material and adjustment of the ore particle size.

In this step, nickel oxide ore is classified using a wet sieve or thelike to separate any foreign material that cannot be leached in aleaching step, ore particles having a particle size that is difficult tobe transported by a pump, and the like.

Usually, the screened particle size is about 2 mm, and ore particleshaving particle sizes greater than that are subjected to a pulverizationtreatment.

A slurry is formed by the ore that has undergone pulverization-screeningtreatment, and then the slurry is settled and concentrated, so that anautoclave-supplied slurry having an adjusted solid concentration in theslurry (hereinafter, referred to as slurry concentration) is prepared.In this regard, it is usually desirable to adjust the slurryconcentration to about 30% to 45% by weight.

The nickel oxide ore that serves as the raw material to be treated bythe hydrometallurgical process of the invention, is composed mainly ofso-called lateritic ore, such as limonite and saprolite.

The nickel content of this lateritic ore is usually 0.8% to 2.5% byweight, and nickel is contained in the form of hydroxide or hydroussilica-magnesia (magnesium silicate) mineral.

Further, the content of iron is 10% to 50% by weight, and iron is mainlyin the form of trivalent hydroxide (goethite); however, some divalentiron is contained in hydrous silica-magnesia mineral or the like.Silicic acid components are contained in silica mineral such ascristobalite (amorphous silica), and hydrous silica-magnesia mineral.

In addition, many of chromium components are contained 1 to 5 wt % aschromite mineral containing iron or magnesium. Furthermore, magnesiacomponents are contained in silica-magnesia mineral that almost do notcontain nickel, which is unweathered and has a high hardness value, inaddition to the hydrous silica-magnesia mineral.

As described above, silica mineral, chromite mineral, andsilica-magnesia mineral are so-called gangue components that almost donot contain nickel, in regard to lateritic ore.

That is, in the ore slurry produced from the ore processing step, ingeneral, chromite that largely affects the abrasion of facilities suchas piping and pumps for the leaching step is included.

Therefore, it is preferable to have chromite separated and recovered inadvance for the ore processing step, from the ore slurry produced in theore processing step.

Here, the distribution state of various components in the ore particlesthat constitute the ore slurry will be described.

In an EPMA observation of nickel oxide ore, a major portion of thechromium content has a high proportion of chromium existing as a singlephase independent of a major portion of the iron content, and there aremany particles having a particle size of 20 μm to 1000 μm.

This indicates that minerals including chromium are contained in largeamounts in particles having a particle size of about 20 μm or more, andminerals including nickel and iron are contained in large amounts inparticles having a particle size of about 20 μm or less.

Therefore, in order to effectively separate and recover chromite fromthe ore slurry, it is essential to set up an appropriate classificationparticle size by slurrifying the ore obtained after eliminating coarseparticles, and crushing nickel oxide ore in this ore slurry so as tohave an appropriate particle size.

The crushed particle size at this time can be determined inconsideration of the original purpose of forming the ore slurry;however, the crushed particle size is preferably about 2 mm or less.

Table 1 shows an example of the ore particle size distribution of theore slurry obtained by crushing the ore to a particle size of about 2 mmor less, and the grades of various components at various particle sizescales.

From Table 1, it is understood that chromium, silicon, magnesium and thelike are concentrated in the coarse particle portion having a particlesize of 75 μm or more. On the other hand, it is understood that iron isconcentrated in the fine particle portion having a particle size of 75μm or less.

TABLE 1 Chemical Particle Partition composition [wt %] size [μm] ratio[wt %] Fe Cr Si Mg −2000 + 1400 0.9 36.0 2.0 14.0 6.0 +850 1.8 37.0 3.013.0 6.0 +355 2.7 33.0 3.0 12.0 5.0  +75 5.3 42.0 5.0 9.0 3.0  −75 89.347.0 3.0 6.0 2.0 Average 100.0 45.7 2.7 6.6 2.0

Next, step (A) is a step of separating and recovering chromite in theore slurry produced from the ore processing step. It is also possible toseparate and remove mineral particles of silica mineral,silica-magnesia, or the like as process intermediates.

Also, the step (A) may be carried out as a process included in the oreprocessing step, or may be carried out subsequently to the oreprocessing step.

The method for step (A) is not particularly limited, and methods usingvarious physical separation means that separate chromite from the oreslurry can be applied. However, among these, in view of the analysis ofthe distribution state of various components in the ore particles thatconstitute the ore slurry, in order to concentrate chromite, forexample, up to 41 wt % Cr2O3, which can be easily recycled, afterseparating and recovering chromite, wet physical separation methodsincluding a specific gravity separation is essential.

That is, as illustrated in Table 1, there are limitations on the gradethat can be concentrated by classification, and not only theclassification but also separation utilizing specific gravity is needed.

The classification particle size in this classification may be anyparticle size as long as the goethite containing nickel of the fineparticle portion can be efficiently separated, and it is preferable thatthe classification particle size be selected preferably in the range of20 μm to 150 μm, and more preferably in the range of 45 μm to 75 μm.

That is, the lower limit of the classification point that can beindustrially implemented is 20 μm in most cases, and when thisclassification particle size is less than 20 μm concentration ofchromite in the coarse particle portion is insufficiently achieved, andalso, nickel in the ore slurry used in the leaching step is lost. On theother hand, when the classification particle size is more than 150 μm,removal of silica mineral, chromite, and silica-magnesia in the fineparticle portion is insufficiently achieved.

Furthermore, the technique for this classification is not particularlylimited, but it is desirable to select cyclone classification that iscapable of processing of large quantities with high performance.

Generally, the specific gravity of chromite is known to be larger thanthat of iron hydroxide such as goethite, and thus, coarse chromitehaving a large specific gravity and fine goethite having a smallspecific gravity can be separated efficiently by a cyclone.

The operation pressure of the cyclone is desirably 0.1 MPa to 0.3 MPawhen the separation performance and the processing speed are taken intoconsideration.

Regarding the shape of the cyclone, it is desirable to adjust the shapesuch that the pulp content of the underflow would be 50 wt % or more.

Furthermore, there are no particular limitations on the pulp content ofthe ore slurry supplied to the cyclone, but the pulp content ispreferably 10 wt % to 30 wt %, and more preferably 15 wt % to 20 wt %.

Separation using a cyclone can be achieved even at a pulp content of 10wt % or less; however, a large amount of water is needed, and it is alsodisadvantageous in precipitate concentration in the subsequent steps.Also, when the pulp content is more than 30 wt %, the viscosity of theslurry increases, and separation may become difficult.

That is, when the pulp content after the ore processing step is set to10 wt % to 30 wt % of the range described above, it is not necessary tosupply water afresh, and a tank for dilution is also not necessary,which is preferable.

As described above, when the pulp content, the cyclone operationpressure, and the cyclone shape are optimized, distribution of chromiteto the overflow can be mostly eliminated, and it is preferable from theviewpoint of chromite recovery.

After goethite that contains nickel is separated and removed as much aspossible by the classification using the cyclone described above,chromite is further concentrated using a specific gravity separationapparatus.

The specific gravity separation apparatus used is not particularlylimited; however, it is preferable to select at least one of a shakingtable, a density separator, and a spiral concentrator, and it is morepreferable to select at least one of a density separator and a spiralconcentrator, which are adequate for processing of large quantities.

In the case of using the spiral concentrator, the pulp content of theslurry supplied to this is preferably more than 15 wt % but less than 35wt %, and more preferably more than 20 wt % but less than 30 wt %.

When the pulp content is 15 wt % or less, the separation performance maybe deteriorated, and when the pulp content is 35 wt % or more, the flowof particles on the chromite concentration side (inner side) is retainedduring the separation with a spiral concentrator, and build-up occurs,so that separation may not be achieved sufficiently.

Furthermore, in the case of using the spiral concentrator, a spiraltreatment is performed several times on chromite (outer side) which isconcentrated to 15 wt % or more and 40 wt % or less and thus therecovery rate of chromite is increased.

In the case of using the density separator, it is desirable to set theamount of teeter water to 0.5 to 7.0 [m3·h−1/m2].

When the amount of teeter water is less than 0.5, the effect of hinderedsettling is small, and specific gravity separation is not carried outefficiently.

On the other hand, when the amount of teeter water is larger than 7.0,even chromite particles are cause to rise up, and a loss may occur onthe overflow side. In this case, the amount of chromite in the slurrysupplied to the leaching step increases, and it becomes disadvantageousfrom the viewpoints of chromite recovery as well as lowering of theCr-grade in hematite.

In addition, when the slurry treatment is performed several times usingthe density separator, the Cr2O3-grade increases.

Furthermore, simply by this specific gravity separation, concentrationcan be achieved to obtain a Cr2O3-grade in chromite of 41 wt % to 50 wt% or more; however, in order to achieve further concentration, it isdesirable to separate and remove hematite that is contained in a traceamount.

Since the specific gravity of removed hematite is very close to thespecific gravity of chromite, magnetic separation is utilized.

On the occasion of magnetic separation, the magnetic field strength isnot particularly limited, and may vary depending on the belt speed, beltthickness, or other apparatuses; however, the magnetic field strength ispreferably in the range of 200 [Oe] to 2000 [Oe].

When the magnetic field strength is less than 200 [Oe], the magneticfield is so weak that separation and elimination of hematite may beachieved insufficiently. On the other hand, when the magnetic fieldstrength is more than 2000 [Oe], there is no problem with the removal ofhematite, but there are occasions in which even chromite is magnetized,and thus the magnetic separation does not work.

Particularly preferably, it is desirable to use a low-magnetic fieldmagnetic concentrator.

[2] Leaching Step

The leaching step is a step of forming a leached slurry composed of aleach residue and a leachate, by adding sulfuric acid to the ore slurryobtained through the ore processing step and step (A), and then stirringthe mixture at a temperature of 220° C. to 280° C. In this step, apreheater, an autoclave, and a flash tank are used as main facilities.

In regard to this leaching step, leaching of nickel, cobalt and the likeas sulfates, and fixation of the leached iron sulfate as hematite areachieved by a leaching reaction represented by the following reactionformulae (1) to (3), and a high temperature thermal hydrolysis reactionrepresented by reaction formulae (4) and (5).

However, since fixation of iron ions does not proceed to completion, theliquid portion of the leached slurry thus obtainable usually containsdivalent and trivalent iron ions in addition to nickel, cobalt and thelike.

[Chemical Formula 1]

[Leaching reaction]

MO+H₂SO₄

MSO₄+H₂O  (1)

(wherein M represents Ni, Co, Fe, Zn, Cu, Mg, Cr, Mn or the like)

2Fe(OH)₃+3H₂SO₄

Fe₂(SO₄)₃+6H₂O  (2)

FeO+H₂SO₄

FeSO₄+H₂O  (3)

[Chemical Formula 2]

[High temperature thermal hydrolysis reaction]

2FeSO₄+H₂SO₄+½O₂

Fe₂(SO₄)₃+H₂O  (4)

Fe₂(SO₄)₃+3H₂O Fe₂O₃+3H₂SO₄  (5)

The reaction temperature for the leaching step is 220° C. to 280° C.,and preferably 240° C. to 270° C.

That is, when the reactions are carried out in this temperature range,iron is fixed as hematite.

When the reaction temperature is lower than 220° C., since the rate ofthe high temperature thermal hydrolysis reaction is slow, iron remainsdissolved in the reaction solution. Therefore, the solution purificationload for removing iron is increased, and it may be very difficult toseparate iron from nickel. On the other hand, when the temperatureexceeds 280° C., the high temperature thermal hydrolysis reaction itselfis accelerated; however, it is difficult to select the material for thevessel used in high-temperature pressure leaching, and the cost forsteam applied to the temperature rise is increased, which is thereforeinappropriate.

The amount of use of sulfuric acid used in the leaching step is notparticularly limited, and an amount slightly excessive compared to thestoichiometric amount required for the iron in the ore to be leached andconverted to hematite, for example, 300 kg to 400 kg per ton of the ore,is used. Particularly, when the amount of addition of sulfuric acid perton of the ore is more than 400 kg, the cost for sulfuric acid and thecost for the neutralizing agent in the subsequent steps are increased,which is not preferable. Furthermore, regarding the amount of use ofsulfuric acid in view of the leaching step product, the concentration offree sulfuric acid at the time of completion of leaching is aimed to be25 g/L to 50 g/L, and preferably, an amount of use of sulfuric acid of35 g/L to 45 g/L is used.

When the above-described conditions are satisfied, the true density ofthe leach residue is increased, a high density leach residue is stablyproduced, and the solid-liquid separability of the slurry is increased.Therefore, simplification of the facilities of the solid-liquidseparation step, which is the subsequent step, can be achieved.

That is, when the concentration is less than 25 g/L, when the slurryincluding the leaching residue is settled, precipitate concentration ofthe solid components is achieved incompletely, and floating solidcomponents remain in the supernatant. This is because the rate of thehigh temperature thermal hydrolysis reaction is slow, dehydration ofiron hydroxide does not proceed sufficiently, and hematite having a lowtrue density is formed.

On the other hand, when the concentration is more than 50 g/L, it isnecessary to enhance durability of the leaching facilities, and sincethe amount of use of the neutralizing agent required for theneutralization of acid is markedly increased, it is disadvantageous inview of cost.

[3] Solid-Liquid Separation Step

The solid-liquid separation step is a step of washing in multiple stagesthe leached slurry formed in the previous leaching step, and obtaining aleachate containing nickel and cobalt, and a leach residue. Thereby,nickel and the like that adhere to the leach residue and are disposedare recovered into the leachate.

[4] Neutralization Step [Step (B-1) and Step (B-2)]

(4-1) Neutralization Step 1 [Treatment of Leachate]

Step (B-1)

This step (B-1) is a step in which a neutralizing agent (pH adjustingagent) is added to adjust the pH to 4 or less, and preferably to be inthe range of 3.2 to 3.8, while oxidation of a leachate 11 obtained inthe leaching step is suppressed by neutralizing the leachate 11 that hasbeen separated in the previous solid-liquid separation step, and aresidue 26 after the step (B-1) as a neutralized precipitate slurrycontaining trivalent iron, and a mother liquor (1) 14 for nickelrecovery are formed.

When this step is used, neutralization of the excess acid used in theleaching step is carried out, and also, removal of trivalent iron ionsremaining in the leachate is carried out.

Upon the neutralization, when the pH exceeds 4, generation of nickelhydroxide is increased.

When a neutralizing agent containing Ca, such as CaCO3, is used, gypsumis produced; however, since the residue 26 after the step (B-1) of theneutralized precipitate slurry generated in this step is such that aportion is returned to the solid-liquid separation step and repeatedlyused, incorporation of gypsum into the leach residue slurry occurs.

Thus, an Mg-based alkali such as Mg(OH)2, which does not contain Ca, oran Mg-based neutralizing agent such as MgO, which dissolves in theleachate and thereby exhibits alkalinity, is used as the neutralizingagent.

[5] Zinc Removal Step

This zinc removal step is a step in which, prior to the step ofseparating nickel and cobalt as sulfides, hydrogen sulfide gas is blowninto the mother liquor obtained in the previous step, sulfidescontaining zinc are produced, and zinc sulfide precipitate slurry and amother liquor for nickel and cobalt recovery are formed.

This is a step of selectively removing zinc by suppressing the rate of asulfurization reaction by generating mild conditions at the time of thesulfurization reaction, and suppressing co-precipitation of nickel thatco-exists at a higher concentration compared with zinc.

This zinc sulfide precipitate slurry thus obtained can be sent to thefinal neutralization step (7) and treated, similarly to the neutralizedprecipitate slurry obtainable in the neutralization step.

[6] Sulfurization Step

This sulfurization step is a step of blowing hydrogen sulfide into themother liquor (2) for nickel and cobalt recovery obtained in the zincremoval step, and producing a mixed sulfide (zinc sulfide precipitate)17 containing nickel and cobalt, and a barren liquor 18.

Here, the barren liquor 18 thus obtained is at a pH of about 1 to 3, andcontains impurities such as iron, magnesium and manganese that arecontained without being sulfurized, as well as slight amounts of nickeland cobalt as a recovery loss. Therefore, the barren liquor 18 is usedas washing water for the leach residue in the solid-liquid separationstep, and as the washing water for the neutralized residue produced inthe neutralization step.

(4-2) Neutralization Step 2 [Treatment of Leach Residue Slurry]

Step (B-2)

This step (B-2) is a step of neutralizing part of the leach residue(leach residue slurry 12) produced in the solid-liquid separation step,using an Mg-based alkali such as Mg(OH)₂ or an Mg-based neutralizingagent such as MgO, and recovering hematite particles.

The method for step (B-2) is not particularly limited, but a Ca-basedalkali is not used as the neutralizing agent. For example, when CaCO₃ isused as the neutralizing agent, this compound reacts with adheringsulfuric acid, and gypsum is produced. Since this gypsum has lowsolubility, gypsum is precipitated as a solid and increases the sulfurgrade in the residue. On the other hand, since MgSO₄ has highsolubility, this compound is not easily precipitated as a solid, and iseffective for the decrease of sulfur.

Therefore, the neutralizing agent is preferably Mg(OH)₂, which is anMg-based alkali; however, an Mg-based neutralizing agent such as MgO₂may be used.

Here, the analysis of the distribution state of various components inthe ore particles that constitute the leach residue slurry 12 will bedescribed.

First, Table 2 shows an example of the ore particle size distribution ofthe leach residue obtained by leaching the ore slurry obtained bycrushing the ore to a particle size of about 2 mm or less, and thegrades of various components at various particle size scales.

TABLE 2 Chemical Particle Partition composition [wt %] size [μm] ratio[wt %] Fe Cr Si Mg −2000 + 1400 0.0 — — — — +850 0.0 — — — — +355 0.128.0 2.0 24.0 0.0  +75 0.7 26.0 7.0 25.0 1.0  −75 99.1 45.0 2.0 8.0 1.0Average 100.0 44.6 2.5 7.8 1.0

From Table 2, it can be seen that iron is concentrated in the fineparticle portion having a particle size of 75 μm or less, and silicon isseparated in this portion. Meanwhile, the analysis of the leach residuewas carried out by removing adhering sulfuric acid by washing the leachresidue slurry with water.

From the results described above, by utilizing the fact that particlescontaining iron at a high content are fine particles than thoseparticles containing chromium, silicon and the like at high contents,the particles containing iron can be separated from the coarse particleportion containing chromium, silicon and the like at high contents, bymeans of screening means such as a classification method, and drivenaway out of the system, and hematite can be recovered as a resource.

The classification method is preferably a treatment using a cyclone orthe like, which is capable of processing of large quantities.

[7] Final Neutralization Step

This final neutralization step is a step of precipitating metal ions inthe liquid as neutralized precipitate and obtaining a final neutralizedresidue 19, by adding a treatment solution 27 after the step (B-2),which is obtained in the step (B-2), the portion of the slurry that hasnot been treated in the step (B-2) in the leach residue slurry 12obtained after the solid-liquid separation step, and the residue 26after the step (B-1), or optionally, a product obtained by slurrifyingthe zinc sulfide precipitate obtainable in the zinc removal step;further adding a limestone slurry and a slaked lime slurry; andadjusting the pH to about 8 or 9. Meanwhile, the final neutralizedresidue 19 thus obtained is stored in the tailings dam 20.

EXAMPLES

Hereinafter, the invention will be further described by way of Examples,but the invention is not intended to be limited to these Examples.

In the Examples, analyses are carried out using a fluorescent X-rayanalysis method or an ICP emission analysis method for the analysis ofmetals.

Example 1

In the production flow of FIG. 1 according to the invention, the step(A) for the ore slurry 9 was performed according to the exemplary flowdiagram illustrated in FIG. 3, the ore slurry 9 was subjected to theclassification treatment with the hydrocyclone, goethite was separatedfrom the ore slurry 9, and then the specific gravity separation 1 wasperformed by combination in the order of a density separator and aspiral concentrator.

Classification of an ore slurry having a composition as indicated inTable 3 was carried out using a hydrocyclone (manufactured by DaikiAtaka Engineering Co., Ltd., Model MD-9) as the classification apparatusused in the step (A).

In Example 1, classification was carried out under the conditions of aslurry concentration of 15 wt %, a slurry temperature set to normaltemperature, and an operation pressure of 0.2 MPa.

The ore slurry composition and the hydrocyclone underflow (U/F)composition are presented together in Table 3. Meanwhile, the unit ofthe following table is percent (%) by weight.

TABLE 3 Cr₂O₃ SiO₂ Fe Ni Ore slurry 2.5 4.4 51.5 1.2 Hydrocyclone 13.56.0 45.2 0.8 U/F Unit: wt %

As can be seen from Table 3, in the coarse particle portion(hydrocyclone U/F) obtained by the hydrocyclone, the level of Cr₂O₃ wasincreased to 13.5 wt % relative to 2.5 wt % in the supplied ore, and thelevel of SiO₂ was increased to 6.0 wt % relative to 4.4 wt % in thesupplied ore; however, the level of Fe was reduced to 45.2 wt % relativeto the iron grade 51.5 wt % in the supplied ore.

From the above, it is understood that silica mineral and chromite areconcentrated and separated in the coarse particle portion byclassification of the ore slurry.

Next, in order to understand the separation due to a density separator,the hydrocyclone U/F (slurry concentration: 33 wt %) was supplied to adensity separator (manufactured by Outotec, Inc., “TANKSIZER TS-Lab”,and having an inner diameter of tank: 228.6 mm).

The supply rate was set to 56 [kg/Hr], and the slurry temperature wasset to normal temperature.

The process was carried out by setting the amount of teeter water atthis time to 6.9 [m3·h−1/m2], and the set point (set value of thedensitometer) to 20.

The compositions of the feed of the density separator (hydrocyclone U/F)and the underflow (density separator U/F) are presented in Table 4.

TABLE 4 Cr₂O₃ SiO₂ Fe Ni Hydrocyclone 13.5 6.0 45.2 0.8 U/F Density 16.91.9 35.2 0.7 separator U/F Unit: wt %

As can be seen from Table 4, in the coarse particle portion (densityseparator U/F) obtained by the density separator, the level of Cr₂O₃ wasincreased to 16.9 wt % relative to 13.5 wt % at the time of theclassification with the hydrocyclone (hydrocyclone U/F). However, thelevel of SiO₂ was reduced to 1.9 wt % relative to 6.0 wt % and the levelof iron was reduced to 35.2 wt % relative to 45.2 wt %.

From the above results, it is understood that silica mineral andchromite are concentrated and separated in the coarse particle portionby the density separator treatment.

Moreover, in order to understand the separation due to a spiralconcentrator, the separation of the hydrocyclone U/F (slurryconcentration: 33 wt %) was performed by a spiral concentrator(manufactured by Outotec, Inc., “MC7000”).

The results are presented in Table 5.

TABLE 5 Cr₂O₃ SiO₂ Fe Ni Hydrocyclone 13.5 6.0 45.2 0.8 underflow U/FConcentrate 41.1 0.5 28.3 0.2 Middling 24.4 1.5 32.5 0.4 Tailing 5.3 4.248.0 1.5 Unit: wt %

As can be seen from Table 5, in the “Concentrate” obtained by the spiralconcentrator, the level of Cr₂O₃ was increased to 41.1 wt % relative to13.5 wt % in the supplied ore.

The level of Cr2O3 was increased to 24.4 wt % in the Middling. On theother hand, the level of Cr2O3 was 5.3 wt % in the Tailing.

From these results, it can be understood that chromite is separated evenby the spiral treatment.

Therefore, then, the density separator U/F (1) (slurry concentration: 75wt %) obtained by the density separator was diluted with water to obtaina slurry concentration of 25 wt % in accordance with the flow in FIG. 3,and was subjected to a separation test using a spiral concentrator(manufactured by Outotec, Inc., “MC7000”).

The test results are presented in Table 6.

TABLE 6 Cr₂O₃ SiO₂ Fe Ni Density 16.9 1.9 35.2 0.7 separator U/FConcentrate 41.2 0.6 28.5 0.3 Middling 24.3 1.6 32.7 0.5 Tailing 5.0 4.548.3 1.7 Unit: wt %

As can be seen from Table 6, in the “Concentrate” obtained by the spiralconcentrator, the level of Cr₂O₃ was increased to 41.2 wt % relative to16.9 wt % in the supplied ore.

The level of Cr₂O₃ was increased to 24.3 wt % in the Middling. On theother hand, the level of Cr₂O₃ was 5.0 wt % in the Tailing.

From these results, it can be understood that chromite is separated bythe spiral treatment.

Next, the “Concentrate” obtained by the spiral test was diluted to aslurry concentration of 20 wt %, and the dilution was supplied to alow-magnetic field magnetic concentrator (manufactured by Outotec, Inc.,“Inprosys benchtop LIMS”) at a supply rate of 45.4 [kg/Hr]. Thus,magnetized material (Mag) and non-magnetized material (Non-mag) wereobtained.

The results are presented in Table 7.

TABLE 7 Cr₂O₃ SiO₂ Fe Ni Feed 41.2 0.6 28.5 0.3 Mag 29.5 0.8 43.7 0.4Non-Mag 45.3 0.6 23.1 0.2 Unit: wt %

As can be seen from Table 7, the Cr₂O₃ obtained by the low-magneticfield magnetic concentration (non-magnetized material/Non-mag) wasincreased to 45.3 wt % relative to 41.2 wt % in the supplied ore. On theother hand, the Fe was reduced to 23.1 wt % from 28.5 wt %.

In contrast, the Fe-grade of the Cr₂O₃ (magnetized material/Mag) was43.7 wt %, which indicates that the Fe-grade was high, it can be seenthat magnetite was separated and removed by magnetic concentration, andthe Cr₂O₃-grade of chromite was increased.

From the above results, it can be said that in the smelting method ofthe present invention described in Example 1, concentration can beachieved to a concentration that surpasses the Cr₂O₃-grade of thechromite commonly available in the market.

The recovery rate of the chromite obtained in Example 1 was 42.5 wt %.

The calculation of the recovery rate was performed by the followingFormula (6):

[Formula 1]

Recovery rate [%]=weight of recovered Cr₂O₃/weight of Cr₂O₃ inintroduced ore  (6)

Example 2

In the production flow of FIG. 1 according to the invention, withrespect to the step (A) for the ore slurry 9, the specific gravityseparation was repeatedly performed two times using the densityseparator as illustrated in the exemplary flow diagram illustrated inFIG. 4, and then the specific gravity separation was performed using thespiral concentrator.

First, classification of ore slurry having a composition as indicated inTable 8 was carried out using a hydrocyclone (manufactured by DaikiAtaka Engineering Co., Ltd., Model MD-9) as the classification apparatusused in the step (A).

In Example 2, the classification was carried out under the conditions ofa slurry concentration of 15% by weight, a slurry temperature set tonormal temperature, and an operation pressure of 0.2 MPa.

The ore slurry composition and the hydrocyclone U/F composition arepresented together in Table 8. The unit of the following table ispercent (%) by weight.

TABLE 8 Cr₂O₃ SiO₂ Fe Ni Ore slurry 2.5 4.4 51.5 1.2 Hydrocyclone U/F13.5 6.0 45.2 0.8 Unit: wt %

As can be seen from Table 8, in the coarse particle portion(hydrocyclone U/F) obtained by the hydrocyclone, the level of Cr₂O₃ wasincreased to 13.5 wt % relative to 2.5 wt % in the supplied ore, and thelevel of SiO₂ was increased to 6.0 wt % relative to 4.4 wt % in thesupplied ore; however, the level of Fe was reduced to 45.2 wt % relativeto the iron grade 51.5 wt % in the supplied ore.

From the above, it is understood that silica mineral and chromite areconcentrated and separated in the coarse particle portion byclassification of the ore slurry.

Next, the hydrocyclone U/F (slurry concentration: 33 wt %) was suppliedto a density separator (manufactured by Outotec, Inc., “TanksizerTS-Lab”, and having an inner diameter of tank: 228.6 mm).

The supply rate was set to 56 [kg/Hr], and the slurry temperature wasset to normal temperature.

The process was carried out by setting the amount of teeter water atthis time to 6.9 [m3·h−1/m2], and the set point (set value of thedensitometer) to 20.

The compositions of the feed of the density separator (1) (hydrocycloneU/F) and the underflow (density separator U/F (1)) are presented inTable 9.

TABLE 9 Cr₂O₃ SiO₂ Fe Ni Hydrocyclone U/F 13.5 6.0 45.2 0.8 Densityseparator 16.9 1.9 35.2 0.7 U/F (1) Unit: wt %

As can be seen from Table 9, in the coarse particle portion (densityseparator U/F (1)) obtained by the density separator (1), the level ofCr₂O₃ was increased to 16.9 wt % relative to 13.5 wt % at the time ofthe classification with the cyclone (HC-U/F). However, the level of SiO₂was reduced to 1.9 wt % relative to 6.0 wt % and the level of iron wasreduced to 35.2 wt % relative to 45.2 wt %.

From the above results, it is understood that silica mineral andchromite are concentrated and separated in the coarse particle portionby the density separator treatment.

The density separator U/F (1) (slurry concentration: 75 wt %) wasdiluted with water to obtain a slurry concentration of 40 wt %, and wasagain subjected to the density separator treatment. The compositions ofthe feed of the density separator (2) (density separator U/F (1)obtained by the first density separator treatment) and the underflow(density separator U/F (2) obtained by the second density separatortreatment) are presented in Table 10.

TABLE 10 Cr₂O₃ SiO₂ Fe Ni Density separator 16.9 1.9 35.2 0.7 U/F (1)Density separator 21.1 1.3 30.6 0.4 U/F (2) Unit: wt %

From Table 10, it can be found that the level of Cr₂O₃ increases from16.9 wt % to 21.1 wt %. Thus, it can be confirmed that the chromiteconcentration is promoted when the treatment with the density separatoris repeatedly performed.

Next, a spiral test was performed in which the density separator U/F (2)(slurry concentration: 75 wt %) obtained by the density separator (2)was diluted with water to obtain a slurry concentration of 25 wt % andwas treated using a spiral separator (a spiral concentrator manufacturedby Outotec, Inc., “MC7000”).

The spiral test results are presented in Table 11.

TABLE 11 Cr₂O₃ SiO₂ Fe Ni Density separator 21.1 1.3 30.6 0.4 U/F (2)Concentrate 44.5 0.4 24.8 0.2 Middling (1) 30.3 1.1 28.4 0.3 Tailing 6.33.0 42.0 1.1 Unit: wt %

As can be seen from Table 11, in the “Concentrate” obtained by thespiral concentrator, the Cr₂O₃ was increased to 44.5 wt % relative to21.1 wt % in the supplied ore. The Cr₂O₃ was increased to 30.3 wt % inthe “Middling”. On the other hand, the Cr₂O₃ was 6.3 wt % in the“Tailing”.

From these results, it can be found that the chromite is separated bythe spiral treatment.

Next, the separation of the Middling (1) having the Cr₂O₃ concentrationof 30.3 wt % was again tried by the spiral treatment. The results arepresented in Table 12.

TABLE 12 Cr₂O₃ SiO₂ Fe Ni Middling (1) 30.3 1.1 28.4 0.3 Concentrate42.5 0.4 24.7 0.2 Middling (2) 19.4 1.8 27.6 0.5 Tailing 19.5 2.2 36.10.7 Unit: wt %

As can be seen from Table 12, in the “Concentrate”, when the Middling(1) was subjected to the spiral treatment again, the Cr₂O₃ was increasedto 42.5 wt % relative to 30.3 wt % in the supplied ore. On the otherhand, the Cr₂O₃ was reduced to 19.4 wt % in a Middling (2) and wasreduced to 19.5 wt % in a Tailing. Preferably, these Middling (2) andTailing are again subjected to the spiral treatment as necessary.

The “Concentrate” obtained by two spiral tests was mixed and diluted toa slurry concentration of 20 wt %, and the dilution was supplied to alow-magnetic field magnetic concentrator (manufactured by Outotec, Inc.,“Inprosys benchtop LIMS”) at a supply rate of 45.4 [kg/Hr]. Thus,magnetized material (Mag) and non-magnetized material (Non-mag) wereobtained. The results are presented in Table 13.

TABLE 13 Cr₂O₃ SiO₂ Fe Ni Feed 44.1 0.4 24.7 0.2 Mag 31.6 0.6 36.6 0.3Non-Mag 48.5 0.4 20.0 0.1 Unit: wt %

As can be seen from Table 13, the Cr₂O₃ obtained by the low-magneticfield magnetic concentration (non-magnetized material/Non-mag) wasincreased to 48.5 wt % relative to 44.1 wt % in the supplied ore. On theother hand, the Fe was reduced to 20.0 wt % from 24.7 wt %.

In contrast, the Fe-grade of the Cr₂O₃ (magnetized material/Mag) was36.6 wt %, which indicates that the Fe-grade was high, it can be seenthat magnetite was separated and removed by magnetic concentration, andthe Cr₂O₃-grade of chromite was increased.

From the above results, it can be said that in the smelting method ofthe present invention described in Example 2, concentration can beachieved to a concentration that surpasses the Cr2O3-grade of thechromite commonly available in the market.

The recovery rate of the chromite obtained in Example 2 was 44%.

The calculation of the recovery rate was performed by Formula (6) as inExample 1.

Comparative Example 1

After a classification treatment was performed using a hydrocycloneaccording to an exemplary flow diagram according to Comparative Example1 illustrated in FIG. 5, the separation was performed using a high-meshseparator according to the size of solids contained in the ore slurry 9,instead of the specific gravity separation treatment in Example 1.

Classification of the ore slurry was carried out using a hydrocyclone(manufactured by Daiki Ataka Engineering Co., Ltd., “Model MD-9”) as theclassification apparatus.

Here, classification was carried out under the conditions of a slurryconcentration of 9.8% by weight, a slurry temperature set to normaltemperature, and an operation pressure of 0.22 MPa.

The hydrocyclone underflow (hydrocyclone U/F) having a slurryconcentration of 33 wt % was diluted to a slurry concentration of 4.9 wt%, and the dilution was charged into a high-mesh separator (manufacturedby Kikosha Co., Ltd., “KUC-612S”).

The supply rate to the high-mesh separator was 0.98 [m3/hour], the speedof rotation of the bucket was 0.8 rpm, the bucket length was 75 mm, andthe bucket had holes having a diameter of 4 mm opened at a pitch of 6mm, while the ratio of hole area was 40%.

The amount of washing water was set to 6 m3/hour.

The compositions of the ore slurry and the hydrocyclone underflow(hydrocyclone U/F) and the composition of the underflow of the high-meshseparator (high-mesh separator U/F) are presented in Table 14.

TABLE 14 Cr₂O₃ Ni Ore slurry 4.1 1.1 Hydrocyclone U/F 13.0 0.8 High-meshseparator U/F 19.1 0.5 Unit: wt %

As is obvious from Table 14, concentration was achieved from theCr₂O₃-grade of the ore slurry of 4.1 wt % to 13.0 wt % in the coarseparticle portion of the hydrocyclone (hydrocyclone U/F), and to 19.1 wt% in the coarse particle portion of the high-mesh separator (high-meshseparator U/F); however, the intended composition level of commerciallyavailable products was not achieved.

In this process, there was no problem in the concentration using ahydrocyclone in particular; however, it can be judged that theconcentration with the high-mesh separator is unsatisfactory.

Thus, the following investigation was carried out to find the cause.

Each of the underflows (hydrocyclone U/F and high-mesh separator U/F)was screened with a sieve having a mesh size of 75 μm, and was subjectedto the analysis based on the above and below values of 75 μm. Theresults presented in Table 15 were obtained.

TABLE 15 Grade [wt %] Distribution Total Total of Size [μm] ratio [wt %]Cr Cr₂O₃ Fe Ni of Cr Cr₂O₃ Hydrocyclone U/F +75 43 16.7 24.4 36.0 0.58.9 13.0 −75 57 3.0 4.4 50.6 1.0 High-mesh +75 85 14.2 20.7 38.1 0.513.1 19.1 separator U/F −75 15 6.7 9.8 49.8 0.7

In Table 15, it was found that the Cr-grade of the underflow of thehigh-mesh separator (high-mesh separator U/F) was 14.2 wt % (20.7 wt %;Cr₂O₃), which was lower than 16.7 wt % (24.4 wt %; Cr₂O₃) of theunderflow of the hydrocyclone (hydrocyclone U/F), and specific gravityseparation was not achieved at all.

From these results, the high-mesh separator performed only the operationof slime removal, and did not perform the operation of specific gravityseparation.

As such, it can be seen that a chromite having a Cr2O3-grade equivalentto the level of commercially available products could not beconcentrated any further, unless specific gravity separation was carriedout.

Example 3

The overflow of the hydrocyclone and the overflow of the densityseparator of Example 1 were introduced into an autoclave at a ratio of77:15 by solid weight, 98% sulfuric acid was added to this mixture, andthe mixture was subjected to high temperature pressurized sulfuric acidleaching under the following conditions. Thus, a leached slurry 10 wasproduced.

Furthermore, the leached slurry thus produced was separated into aleachate 11 and a leach residue slurry 12 by a solid-liquid separationstep.

[Leaching Conditions]

Leaching temperature: 245° C.

Leaching time: 60 minutes

Final free sulfuric acid concentration (at the time of completion ofleaching): 40 [g/L]

Slurry concentration: 30 wt %

Autoclave volume: 5 L

Next, in order to find the Cr₂O₃-grade in the leach residue slurry 12,Mg(OH)₂ slurry as a neutralizing agent at a concentration of 20 wt % wasadded to the leach residue slurry 12, and the leach residue slurry wasneutralized at 70° C. to obtain a pH of 2.5.

Subsequently, this slurry was subjected to solid-liquid separation usinga 5 C filter paper. The Mg(OH)₂ slurry was further added thereto untilthe slurry reached pH 6, and then the slurry was further subjected tosolid-liquid separation using the 5 C filter paper.

The Cr₂O₃-grade of the final neutralized residue thus obtained was 0.9wt %. Since the solubility of MgSO₄ to be produced was high, thesulfur-grade of the residue was 0.53 wt %.

Comparative Example 2

The ore slurry of Example 1 was treated in the same manner as in Example3, except that the ore slurry was introduced into an autoclave withouttreating the slurry with a hydrocyclone and a density separator. TheCr₂O₃-grade of the final neutralized residue thus obtained was 2.1 wt %.

Since the solubility of MgSO4 to be produced was high, the sulfur-gradeof the residue was 0.53 wt %.

As is obvious from a comparison between Example 3 and ComparativeExample 2, when the ore slurry was first classified with thehydrocyclone and then treated with the density separator, which is oneof specific gravity separation apparatuses, chromite in the ore slurrycould be separated and removed, and the Cr₂O₃-grade in the residue couldbe halved.

Comparative Example 3

A leach residue slurry 12 was produced in the same manner as in Example3, slaked lime slurry at a concentration of 25 wt % was added as aneutralizing agent to the entire amount of the leach residue slurry, andthe slurry was neutralized to pH 8.5 at 60° C. Metal ions wereprecipitated as precipitate, and a neutralized residue and a treatmentsolution after neutralization were obtained by solid-liquid separation.

This neutralized residue was subjected to cyclone classification, andthus hematite 28 was separated.

A mixed liquid was prepared by mixing the remaining neutralized residuefrom which hematite 28 had been separated with the treatment solutionafter neutralization, and a slaked lime slurry at a concentration of 25wt % was added thereto. Thereafter, solid-liquid separation was repeatedusing a 5 C filter paper, and thus a final neutralized residue wasobtained.

The Cr2O3-grade of the final neutralized residue thus obtained was 0.8wt %. Since the solubility of CaSO₄ to be produced was small, thesulfur-grade of the residue was 5.72 wt %, and the Ca-grade was 8.49 wt%.

Comparative Example 4

As illustrated in an exemplary flow diagram according to ComparativeExample 4 illustrated in FIG. 6, a separation test was performed underthe same condition as that of Example 1, except that the ore slurry 9was subjected to the specific gravity separation in the same manner asin Example 1 without the classification treatment by the hydrocycloneand the classification treatment was finally performed by thehydrocyclone.

Table 16 indicates results obtained in such a manner that the ore whichwas not subjected to the classification treatment by the hydrocyclonewas subjected to a specific gravity separation treatment using a densityseparator.

The viscosity of a feed (ore slurry) was high, but the concentration ofCr₂O₃ subjected to the classification treatment by the density separatorwas not as high as the underflow (see a density separator U/F in Table6) subjected to feeding.

TABLE 16 Cr₂O₃ SiO₂ Fe Ni Feed 2.5 4.4 51.5 1.2 Density separator 9.52.5 40.2 1.0 U/F Unit: wt %

The results of the separation of the density separator U/F with a spiralseparator are presented in Table 17.

As is obvious from Table 17, even when the specific gravity separationtreatment was performed by the spiral separator, the Cr2O3 concentrationwas 25.3 wt %, but did not exceed 41 wt % or more.

This is considered that the slurry viscosity is high and the effect ofthe spiral cannot be exhibited because coarse particles and fineparticles are not separated by the density separator.

TABLE 17 Cr₂O₃ SiO₂ Fe Ni Density separator 9.5 2.5 40.2 1.0 U/FConcentrate 25.3 0.8 32.6 0.5 Middling 13.6 2.1 37.3 0.8 Tailing 2.8 5.955.2 2.6 Unit: wt %

Subsequently, the classification treatment was performed by thehydrocyclone.

As indicated in Table 18, the concentration of Cr₂O₃ was 35.3 wt %, butdid not meet 41 wt % or more.

TABLE 18 Cr₂O₃ SiO₂ Fe Ni Concentrate 25.3 0.8 32.6 0.5 Hydrocyclone U/F35.3 2.5 28.6 0.2 Unit: wt %

The concentration could be achieved up to a concentration above theCr₂O₃ grade of the chromite which was commercially available in themarket. From this fact, it can be found that it is important that thecyclone classification is performed first to remove microparticles.

As is obvious from the above results, the hydrometallurgical process fornickel oxide ore of the present invention is suitable as a smeltingmethod based on high pressure leaching that is used in thehydrometallurgical field of nickel oxide ore.

REFERENCE SIGNS LIST

-   -   8: Nickel oxide ore    -   9: Ore slurry    -   10: Leached slurry    -   11: Leachate    -   12: Leach residue slurry    -   14: Mother liquor (1)    -   15: Zinc sulfide precipitate    -   16: Mother liquor (2)    -   17: Ni—Co mixed sulfide    -   18: Barren liquor    -   19: Final neutralized residue    -   20: Tailings dam    -   22: Autoclave-supplied slurry    -   23: Chromite    -   26: Residue after step (B-1)    -   27: Treatment solution after step (B-2)    -   28: Hematite

DRAWINGS

-   -   FIG. 1    -   8 NICKEL OXIDE ORE    -   [1] ORE PROCESSING STEP    -   9 ORE SLURRY    -   STEP (A)    -   22 AUTOCLAVE-SUPPLIED SLURRY    -   23 CHROMITE    -   [2] LEACHING STEP    -   10 LEACHED SLURRY    -   [3] SOLID-LIQUID SEPARATION STEP    -   11 LEACHATE    -   STEP (B-1)    -   26 RESIDUE AFTER STEP (B-1)    -   14 MOTHER LIQUOR (1)    -   [5] ZINC REMOVAL STEP    -   15 ZINC SULFIDE PRECIPITATE    -   16 MOTHER LIQUOR (2)    -   [6] SULFURIZATION STEP    -   17 Ni—Co MIXED SULFIDE    -   18 BARREN LIQUOR    -   12 LEACH RESIDUE SLURRY    -   STEP (B-2)    -   27 TREATMENT SOLUTION AFTER STEP (B-2)    -   28 HEMATITE    -   [7] FINAL NEUTRALIZATION STEP    -   19 FINAL NEUTRALIZED RESIDUE    -   20 TAILINGS DAM    -   FIG. 2    -   8 NICKEL OXIDE ORE    -   (1) ORE PROCESSING STEP    -   9 ORE SLURRY    -   (2) LEACHING STEP    -   10 LEACHED SLURRY    -   (3) SOLID-LIQUID SEPARATION STEP    -   11 LEACHATE    -   (4) NEUTRALIZATION STEP    -   13 NEUTRALIZED PRECIPITATE SLURRY    -   14 MOTHER LIQUOR (1)    -   (5) ZINC REMOVAL STEP    -   15 ZINC SULFIDE PRECIPITATE    -   16 MOTHER LIQUOR (2)    -   (6) SULFURIZATION STEP    -   17 Ni—Co MIXED SULFIDE    -   18 BARREN LIQUOR    -   12 LEACH RESIDUE SLURRY    -   (7) FINAL NEUTRALIZATION STEP    -   19 FINAL NEUTRALIZED RESIDUE    -   20 TAILINGS DAM    -   FIG. 3    -   1 ORE SLURRY    -   2 HYDROCYCLONE    -   3 HYDROCYCLONE U/F    -   4 HYDROCYCLONE O/F    -   5 DENSITY SEPARATOR (1)    -   [2] TO LEACHING STEP    -   6 DENSITY SEPARATOR U/F (1)    -   7 DENSITY SEPARATOR O/F    -   [2] TO LEACHING STEP    -   8 SPIRAL CONCENTRATOR (1)    -   [2] TO LEACHING STEP    -   [2] TO LEACHING STEP    -   9 FEED    -   10 LOW-MAGNETIC FIELD MAGNETIC CONCENTRATION    -   [2] TO LEACHING STEP    -   11 CHROMITE (PRODUCT)    -   FIG. 4    -   1 ORE SLURRY    -   2 HYDROCYCLONE    -   3 HYDROCYCLONE U/F    -   4 DENSITY SEPARATOR (1)    -   5 DENSITY SEPARATOR U/F (1)    -   6 DENSITY SEPARATOR (2)    -   7 DENSITY SEPARATOR U/F (2)    -   8 SPIRAL CONCENTRATOR (1)    -   9 FEED    -   10 LOW-MAGNETIC FIELD MAGNETIC CONCENTRATION    -   [2] TO LEACHING STEP    -   11 HYDROCYCLONE O/F    -   [2] TO LEACHING STEP    -   12 DENSITY SEPARATOR O/F    -   [2] TO LEACHING STEP    -   13 DENSITY SEPARATOR O/F    -   [2] TO LEACHING STEP    -   14 SPIRAL CONCENTRATOR (2)    -   [2] TO LEACHING STEP    -   [2] TO LEACHING STEP    -   15 CHROMITE (PRODUCT)    -   FIG. 5    -   1 ORE SLURRY    -   2 HYDROCYCLONE    -   3 HYDROCYCLONE U/F    -   4 HIGH-MESH SEPARATOR    -   5 HIGH-MESH SEPARATOR U/F    -   6 CHROMITE CONCENTRATE    -   7 HYDROCYCLONE O/F    -   8 TO LEACHING STEP    -   9 HIGH-MESH SEPARATOR O/F    -   8 TO LEACHING STEP    -   FIG. 6    -   1 ORE SLURRY    -   2 DENSITY SEPARATOR (1)    -   3 DENSITY SEPARATOR U/F (1)    -   4 SPIRAL CONCENTRATOR (1)    -   5 HYDROCYCLONE    -   6 HYDROCYCLONE U/F    -   7 CHROMITE CONCENTRATE    -   8 DENSITY SEPARATOR O/F    -   9 TO LEACHING STEP    -   9 TO LEACHING STEP    -   9 TO LEACHING STEP    -   10 HYDROCYCLONE O/F    -   9 TO LEACHING STEP

1. A hydrometallurgical process for nickel oxide ore of recoveringnickel and cobalt using a high pressure acid leaching that includes anore processing step, a leaching step, a solid-liquid separation step, aneutralization step, a zinc removal step, a sulfurization step, and afinal neutralization step, the process comprising at least one stepselected from the following step (A), step (B-1), and step (B-2): Step(A): separating and recovering chromite particles in an ore slurryproduced in the ore processing step, by a recovery process including aspecific gravity separation; Step (B-1): neutralizing a leachate with amagnesium-based neutralizing agent, the leachate being produced bysubjecting the ore slurry that has a chromium grade lowered through thestep (A), to the leaching step and the solid-liquid separation step; andStep (B-2): neutralizing a leach residue slurry with a magnesium-basedneutralizing agent to recover hematite particles, the leach residueslurry being produced by subjecting the ore slurry that has a chromiumgrade lowered through the step (A), to the leaching step and thesolid-liquid separation step.
 2. A hydrometallurgical process for nickeloxide ore of recovering nickel and cobalt from nickel oxide ore using ahigh pressure acid leaching that includes an ore processing step, aleaching step, a solid-liquid separation step, a neutralization step, azinc removal step, a sulfurization step, and a final neutralizationstep, the process comprising the following step (A), step (B-1), andstep (B-2): Step (A): separating and recovering chromite particles in anore slurry produced in the ore processing step, by a recovery processincluding a specific gravity separation; Step (B-1): neutralizing aleachate with a magnesium-based neutralizing agent, the leachate beingproduced by subjecting the ore slurry that has a chromium grade loweredthrough the step (A), to the leaching step and the solid-liquidseparation step; and Step (B-2): neutralizing a leach residue slurrywith a magnesium-based neutralizing agent to recover hematite particles,the leach residue slurry being produced by subjecting the ore slurrythat has a chromium grade lowered through the step (A), to the leachingstep and the solid-liquid separation step.
 3. The hydrometallurgicalprocess for nickel oxide ore of claim 2, wherein the recovery process ofthe step (A) includes subjecting the ore slurry to cycloneclassification, reducing fine iron hydroxide particles, and thenrecovering chromite particles in the ore slurry from the ore slurry as aconcentrate of chromite using the specific gravity separation.
 4. Thehydrometallurgical process for nickel oxide ore of claim 3, wherein therecovery process of the step (A) includes performing cycloneclassification without diluting a slurry concentration of the oreslurry.
 5. The hydrometallurgical process for nickel oxide ore of claim2, wherein the recovery process of the step (A) includes collectingchromite into an underflow in cyclone classification in the entireamount except for unavoidable losses.
 6. The hydrometallurgical processfor nickel oxide ore of claim 2, wherein the specific gravity separationincludes at least one step of selected from a step of using a densityseparator and a step of using a spiral concentrator.
 7. Thehydrometallurgical process for nickel oxide ore of claim 6, wherein thestep of using the density separator is performed two times or more onthe slurry concentrated, using the density separator.
 8. Thehydrometallurgical process for nickel oxide ore of claim 6, wherein thestep of using the spiral concentrator is performed two times or more onthe slurry concentrated, using the spiral concentrator.
 9. Thehydrometallurgical process for nickel oxide ore of claim 6, wherein apulp content of the slurry supplied to the spiral concentrator is 15 wt% solids to 35 wt % solids.
 10. The hydrometallurgical process fornickel oxide ore of claim 6, wherein an amount of teeter water suppliedto the density separator is 0.5 to 7.0 [m³·h⁻¹/m²].
 11. Thehydrometallurgical process for nickel oxide ore of claim 2, whereinafter the specific gravity separation, the slurry is subjected to amagnetic separation, which is a physical separation, to remove magnetiteso as to recover non-magnetized material as a chromite concentrate. 12.The hydrometallurgical process for nickel oxide ore of claim 2, whereinin the step (B-2), a pH of the leach residue slurry neutralized isadjusted to be in a range of 4 to 7, and final neutralization is thencarried out using an alkali other than a magnesium-based neutralizingagent.
 13. The hydrometallurgical process for nickel oxide ore of claim2, wherein in the step (B-2), the leach residue slurry or a neutralizedresidue slurry including the leach residue slurry is subjected tocyclone classification, and a fine particle portion thus classified isrecovered as a concentrate of hematite.
 14. The hydrometallurgicalprocess for nickel oxide ore of claim 2, wherein the ore processing stepis a step of performing removal of foreign material in mined rawmaterial ore and adjustment of a particle size of the ore to form oreslurry; the leaching step is a step of adding sulfuric acid to the oreslurry and stirring the mixture at a high temperature and a highpressure to form a leached slurry composed of a leach residue and theleachate; the solid-liquid separation step is a step of washing theleached slurry in multiple stages to obtain the leachate containingnickel and cobalt, and the leach residue slurry; the neutralization stepis a step of adding an alkali to the leachate to form a neutralizedprecipitate slurry containing trivalent iron, and a mother liquor fornickel recovery; the zinc removal step is a step of blowing in hydrogensulfide gas into the mother liquor to form a zinc sulfide precipitateslurry and a mother liquor for nickel and cobalt recovery; thesulfurization step is a step of blowing in hydrogen sulfide into themother liquor for nickel and cobalt recovery, and producing a mixedsulfide containing nickel and cobalt, and a barren liquor; and the finalneutralization step is a step of adding an excess of the barren liquorto the leach residue slurry and adjusting the pH of the mixture to be ina range of 8 to 9, to obtain a final neutralized residue.
 15. Thehydrometallurgical process for nickel oxide ore of claim 2, whereinadjustment of a particle size of the ore in the ore processing step iscarried out by screening to a particle size of 2 mm or less.
 16. Thehydrometallurgical process for nickel oxide ore of claim 2, wherein agrade of chromium(III) oxide in concentrated chromite is 41 wt % ormore.
 17. The hydrometallurgical process for nickel oxide ore of claim1, wherein the recovery process of the step (A) includes subjecting theore slurry to cyclone classification, reducing fine iron hydroxideparticles, and then recovering chromite particles in the ore slurry fromthe ore slurry as a concentrate of chromite using the specific gravityseparation.
 18. The hydrometallurgical process for nickel oxide ore ofclaim 1, wherein the recovery process of the step (A) includescollecting chromite into an underflow in cyclone classification in theentire amount except for unavoidable losses.
 19. The hydrometallurgicalprocess for nickel oxide ore of claim 1, wherein the specific gravityseparation includes at least one step of selected from a step of using adensity separator and a step of using a spiral concentrator.
 20. Thehydrometallurgical process for nickel oxide ore of claim 1, wherein theore processing step is a step of performing removal of foreign materialin mined raw material ore and adjustment of a particle size of the oreto form ore slurry; the leaching step is a step of adding sulfuric acidto the ore slurry and stirring the mixture at a high temperature and ahigh pressure to form a leached slurry composed of a leach residue andthe leachate; the solid-liquid separation step is a step of washing theleached slurry in multiple stages to obtain the leachate containingnickel and cobalt, and the leach residue slurry; the neutralization stepis a step of adding an alkali to the leachate to form a neutralizedprecipitate slurry containing trivalent iron, and a mother liquor fornickel recovery; the zinc removal step is a step of blowing in hydrogensulfide gas into the mother liquor to form a zinc sulfide precipitateslurry and a mother liquor for nickel and cobalt recovery; thesulfurization step is a step of blowing in hydrogen sulfide into themother liquor for nickel and cobalt recovery, and producing a mixedsulfide containing nickel and cobalt, and a barren liquor; and the finalneutralization step is a step of adding an excess of the barren liquorto the leach residue slurry and adjusting the pH of the mixture to be ina range of 8 to 9, to obtain a final neutralized residue.